<!DOCTYPE html>
<html>
<body>
<script>
count = 0;
let observer = new MutationObserver(() => { count++; if (count==1) { console.log(42); while(true){} } });
observer.observe(document.body, {childList: true});
</script>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<b>0</b><b>1</b><b>2</b><b>3</b><b>4</b><b>5</b><b>6</b><b>7</b><b>8</b><b>9</b>
<script>
  // Remove this script block to prevent parser from yeilding, so the parsing work can be done in one task, then the mutation observer handler just runs once.
</script>
</body>
</html>
